A recent study showed that normal weight people have more Bacteroidetes bacteria in their stomach and intestines, while obese people have more Firmicutes bacteria.
Researchers don’t know if Firmicutes bacteria cause obesity or if obesity causes Firmicutes to grow, but the implications are huge: change the bacteria in your gut to change your body!
In fact, the same researchers have found that everyone’s bodies actually extract different calorie amounts from the same exact food, depending on your own particular combination of intestinal microflora.
For example, you and your friend could eat the same steak dinner, but depending on your gut flora, you could extract more calories from the same meal. Google Fermicutes and read up.